In the manufacture of glass products, for example, blown glass containers, it is customary to provide a relatively narrow, elongate conditioning chamber, usually called a forehearth, to cool the glass to a proper relatively uniform temperature as it flows from a glass melting furnace to a forming machine. A typical forehearth is in the shape of an insulated, horizontal trough and is provided with an insulated roof structure.
It is known that the temperature of the glass flowing through a forehearth tends to be nonuniform across the width of the forehearth due to the fact that the loss of heat through the sides of the forehearth tends to make the edge portions of the glass stream in the forehearth cooler than the central portion of the glass stream. For this reason, it is known to provide burners or other heating means to impart heat to the edge portions of the glass stream in the forehearth and to provide means for cooling the central portion of the glass streams in the forehearth relative to the side portions.
The cooling of a modern glass forehearth typically involves the flowing of ambient air along the center line of the forehearth and above the glass in the forehearth, either out of physical contact with the glass in the forehearth, in which case the forehearth is often referred to as an indirectly cooled forehearth, or in contact with the glass in the forehearth, in which case the forehearth is known as a directly cooled forehearth. U.S. Pat. No. 4,680,051 (Blumenfeld et al.) and U.S. Pat. No. 5,169,424 (Grinnen et al.) disclose forehearths of an indirectly cooled nature, and U.S. Pat. No. 4,511,385 (Barkhau et al.) and U.S. Pat. No. 3,999,972 (Brax) disclose forehearths of the directly cooled type. The disclosure of each of the foregoing U.S. patents is incorporated by reference herein.
Cooling of the glass in a forehearth of either the directly cooled or indirectly cooled type tends to be predominantly by radiation from the warm upper surface of the glass stream in the forehearth to the cooler inside surface of the roof structure immediately above center of the forehearth. The cooling air, in a directly cooled forehearth, directly cools the radiation energy receiving surface of the forehearth roof structure, whereas the cooling air in an indirectly cooled forehearth cools a surface that is separated from the radiation receiving surface by a finite thickness of a refractory or other material of only a moderately conductive nature. Thus, the temperature control system for controlling the cooling in a directly cooled forehearth is capable of responding more rapidly to correct glass temperature conditions when they depart from predetermined ideal temperature conditions. Glass temperature uniformity is especially important in modern glass container forming operations, where glass gob weight is very important for quality and glass container volume control, because the weight of a glass gob from which a container is formed is dependent on the viscosity of the glass stream from which the gob is formed, and the viscosity of a glass stream is a function of its temperature.
The roof structure of a directly cooled forehearth of the type illustrated in the aforesaid U.S. Pat. No. 4,511,385 is a very complex structure, being formed from a longitudinally extending series of elements each of which consists of a transverse array of a plurality of individual pieces. The pieces in each transverse array collectively are provided with a complex shape, in a transverse direction, in order to define longitudinally extending barriers to confine the flow of cooling air to the central portion of the forehearth and the flow of products of combustion from the side heating burners to the side regions of the forehearth. Such a multiple piece roof structure is difficult to install, and there is a tendency for the individual pieces to shift with respect to one another over a period of time, thus opening up cracks or spaces between the adjacent surfaces of adjacent blocks in the roof structure. Further, for optimum control of the temperature uniformity of the glass stream within a forehearth, it is desirable to control the firing of the burners on each side of the forehearth independently of the burners on the other side of the forehearth, and known directly cooled forehearths were not provided with such independent firing control systems. This factor is especially important in modern forehearth installations, which tend to be wider than their earlier counterparts to provide longer glass residence times.